1. Field of the Invention
The present invention relates generally to optical amplifiers, and more particularly to an optical amplifier that automatically compensates for wavelength dispersion caused in a WDM (Wavelength Division Multiplexing) system.
2. Description of the Related Art
An optical amplifier employed in a WDM system is required to amplify multiple signals at a time. Further, an ADM (Add/Drop Multiplexer) capable of extracting and inserting individual signal lights independently has been put to practical use. A change in the number of signal wavelengths in the ADM causes a change in signal light power transmitted through an optical fiber.
Even when the number of signal wavelengths is changed, the output power of each signal wavelength can be expected to be constant by controlling an amplifier gain to a constant level. This control method is called AGC (Automatic Gain Control). It is required to increase the response speed of the AGC control of an optical amplifier as the speed of increasing and decreasing the number of wavelengths in the ADM increases.
FIG. 1 is a block diagram showing a conventional optical amplifier. Referring to FIG. 1, a part of input light from an optical fiber 10 is extracted at a light branch part 11 to be fed to a photodiode 13 of an input monitoring part 12, where photoelectric conversion is performed thereon. An input light intensity detection signal output from the photodiode 13 is amplified in a monitoring circuit 14 to be fed to a gain control circuit 16 in a gain control part 15.
Likewise, a part of the output light of an optical fiber 20 is extracted at a light branch part 21 to be fed to a photodiode 23 of an output monitoring part 22, where photoelectric conversion is performed thereon. An output light intensity detection signal output from the photodiode 23 is amplified in a monitoring circuit 24 to be fed to the gain control circuit 16 in the gain control part 15.
The gain control circuit 16 generates a control signal so that the ratio of the input light intensity detection signal to the output light intensity detection signal is constant, and feeds the generated control signal to a laser diode 17. The laser diode 17 outputs pump light having intensity corresponding to the control signal. The pump light is fed through a light composition part 18 to an EDFA (Erbium Doped Fiber Amplifier) 26 forming an amplification part 25, in which the input light is amplified.
Japanese Laid-Open Patent Application No. 9-244080 discloses the technique of monitoring the number of input channels from input light power, controlling a pump light source so that output light power is constant, and making the output light power of each channel constant by switching, in accordance with a change in the number of input channels, set values for controlling the output light power to a constant level.
In order to support a fast increase and decrease in the number of wavelengths, it is necessary to minimize gain variation even if the number of wavelengths changes by causing light monitoring or AGC control to be performed at high speed. In general, the speed of increasing and decreasing the number of wavelengths varies from a few to several hundred microseconds (μs). Therefore, it is necessary to set the response speed of AGC control also to a few to several hundred μs. Further, in the future, the speed of optical switching is expected to increase, so that the speed of increasing and decreasing the number of wavelengths and the response speed may be less than 1 μs.
On the other hand, in the case of operating a small number of wavelengths and transmitting a low-speed signal, the light average power to be monitored may vary because of the signal pattern effect. In SONET (Synchronous Optical Network) or SDH (Synchronous Digital Hierarchy), a 72-bit sequence of consecutive identical digits is caused as a result of scrambling. For example, according to ITU-T G.957, 72 bits of identical digits are recommended as a test pattern for identical digit immunity. For example, if a 72-bit consecutive identical digit pattern is employed for a 155 Mbps signal, the length (time) of the identical digits is approximately 0.46 μs, which is long enough for responding if the response speed of AGC control is a few μs.
Further, even if the input of an optical amplifier is a signal component by 100%, the variation component due to signal pattern is not always the same in the input part and the output part because the output of the optical amplifier includes an ASE (Amplified Spontaneous Emission) component. For example, if 72 bits of consecutive 1s appear in a signal of a mark ratio of 50%, the monitoring result of an input signal is as indicated by the solid line in FIG. 2, and the monitoring result of an output signal at this point is expected to be as indicated by the solid line in FIG. 3. In FIG. 3, a signal component is superposed on an ASE component.
Referring to FIGS. 2 and 3, the ratio of the output y1 of the monitoring circuit 24 to the output x1 of the monitoring circuit 14 in the signal of a mark ratio of 50% is (y1/x1), while the ratio of the output y2 of the monitoring circuit 24 to the output x2 of the monitoring circuit 14 in the signal of a mark ratio of 100% is (y2/x2), which is less than (y1/x1) [(y1/x1)>(y2/x2)].
However, gain control causes the gain of the monitoring results of the input and output to be constant. That is, the gain control part 15 performs AGC control so that the ratio of the output of the monitoring circuit 24 to the output of the monitoring circuit 14 is constant at (y1/x1). Accordingly, the actually obtained monitoring result of the output is as indicated by the solid line in FIG. 4. In FIG. 4, the ratio of the output z2 of the monitoring circuit 24 to the output z1 of the monitoring circuit 14 in the signal of a mark ratio of 100% is (z2/x2), which is equal to (y1/x1) [(z2/x2)=(y1/x1)]. This causes a problem in that the ASE component increases in the part of the signal of a mark ratio of 100%, thus causing a variation in signal gain.